The technical field of this invention is that of nondestructive materials characterization, particularly quantitative, model-based measurement of bulk material and surface condition for flat and curved parts. Measurement of bulk material condition includes measurement of changes in material state caused by fatigue damage, plastic deformation assessment, residual stress, applied loads, and processing conditions. It also includes measurement of material states such as porosity, alloy type, moisture content and temperature. Measurement of surface condition includes roughness, displacement of relative position, coating thickness and coating condition. Each of these also include detection of electromagnetic property changes associated with the presence of single or multiple cracks.
The methods for measurement include the use of quasistatic fields, electromagnetic (magnetometers and dielectrometer) or thermal, that are described by diffusion equations, as opposed to alternative techniques that are described by wave equations. For example, magnetometers have been used to measure foil thicknesses, characterize coatings, and measure porosity, as well as to measure material property profiles as a function of depth into a part, as disclosed in U.S. Pat. Nos. 5,015,951 and 5,453,689. Dielectrometer with multiple electrode spacings have been used to measure profiles of properties such as moisture and porosity using methods disclosed in U.S. Pat. No. 4,814,690.
The present invention relates to a novel apparatus for carrying a test circuit such as a magnetometer or dielectrometer to enable that test circuit to conform to a test surface and to novel meandering winding magnetometer (MWM) test circuits.
In accordance with one aspect of the invention, a sensing apparatus comprises a test circuit which imposes a magnetic field on a test substrate and senses a response of the test substrate. The preferred test circuit is a meandering winding magnetometer in which the winding imposes a magnetic field having a periodic spatial wavelength. The test circuit is supported on a flexible carrier. The carrier protrudes from a case such that the test circuit is exposed to a test substrate and is slidable into the case. The case has an abutment for pressing the flexible carrier and the test circuit against the test substrate as the carrier slides into the case, and the carrier flexes to conform to the test substrate. Preferably, the circuit is formed on a flexible membrane which is retained against the surface of the flexible carrier, and the carrier is supported by a rigid holder. The holder is itself slidable within the case and retains the carrier at its center as the abutment abuts opposite sides of the carrier. Preferably, the holder is pivotable about an axis which extends parallel to the side to side dimension.
In accordance with another aspect of the invention, a test circuit comprises a meandering primary winding for imposing a magnetic field in a test substrate, the winding having extended portions across a sensing region and connecting portions joining adjacent extended portions. At least one meandering sensing winding parallels the primary winding. Terminal leads provide a pair of analyzer connections to each of the primary and sensing windings. First and second leads from the at least one sensing winding are closely parallel to each other at a terminal end thereof, and one of the first and second leads includes a return length which closely parallels outer portions of the sensing winding, that is a first extended length of the winding and the connecting portions of the sensing winding. The leads should be spaced no more than one quarter wavelength from each other and the connecting portions.
A second sensing winding may be provided on an opposite side of the primary winding, and the lead pairs from the respective sensing windings are spaced substantially the same distance, at least one wavelength, to opposite sides of a primary winding lead. A second primary winding lead is displaced from the winding array and from the sensing leads by at least one wavelength. Preferably, the connecting portions of the sensing windings and the leads are covered by at least one shield in a plane parallel to the test circuit, in the area outside the sensing region only.
In accordance with another aspect of the invention, another test circuit comprises a meandering primary winding for imposing a spatially periodic magnetic field in a test substrate, the winding having extended portions across a sensing region and coupling portions joining adjacent extended portions. A plurality of individual sensing coils are positioned between respective adjacent extended portions of the primary winding. In one form, a single loop sensing coil is positioned between each pair of extended portions, and in another form, a meandering sensing winding is positioned to one side of the primary winding and individual loops are positioned to the other side of the primary winding. In a third form several meandering sensing windings are included, each covering more than half a wavelength within the sensing region. As taught in U.S. Pat. No. 5,453,689, the response of a substrate to the imposed field is modelled to provide estimates of the substrate properties. The loops are preferably spaced at least one wavelength from the connecting portions of the primary winding to minimize unmodelled coupling to regions of the primary winding. Preferably, a shield in a plane parallel to the test circuit covers the ends of the sensing coils and primary winding.
The test circuit may be formed on a roll of adhesive tape which may be cut at appropriate lengths for an appropriate number of winding turns and sensing coils for particular application. Bonding pads, such as solder pads, are provided to each sensing coil and on each turn of the primary winding. Extra pads may be provided for connections to shield planes and the like.
Either test circuit may be scanned across a test surface. For improved resolution during scanning, adjacent test circuits may be provided, offset by one quarter wavelength.